It is well known that component fatigue life is a major factor in the design of many mechanical, fluid, and electrical devices and systems. In systems that require high reliability, component fatigue life becomes even more critical. Aircraft gas turbine engines are an example of systems that require high reliability and for which component fatigue life is a critical factor. Component fatigue life is especially critical for the high energy rotor components of gas turbine engines because they pose a significant threat to aircraft safety should an uncontained failure occur. Given the potentially disastrous consequences of hardware failures in any aircraft gas turbine engine, the aircraft engine industry has developed very sophisticated design methodologies which attempt to insure that all critical engine components can reliably meet service life expectations for a given set of operating conditions. Thus, fatigue failure due to repeated duty cycles is a failure mode of great interest to the engine designers because it directly influences the reliability and life cycle cost of the end product. Accordingly, there is heavy emphasis placed upon designing components which safely maximize their fatigue life.
The primary approach to designing fatigue susceptible hardware which has evolved over many years within the aircraft engine community is commonly referred to as the "safe life" method. The safe life method is based on the principle that the minimum number of load cycles that can be sustained before the generation of a fatigue crack or other fatigue indication may be deterministically calculated for any given design. This minimum number of load cycles must take into account variations in hardware dimensions, material properties and operating environments (ambient conditions). In the safe life method, once this minimum number of operating cycles is determined, a retirement limit or life for the hardware is established. Retirement limits are typically set less than the minimum number of load cycles to provide a margin of safety. At least one engine manufacturer has typically used a safety factor of three to determine its retirement limits. It would be understood by those skilled in the art that the safe life method may be applied to any mechanical, electrical or fluid component.
One typical procedure for determining the safe life of a new component for a gas turbine engine involves the following:
Determine the expected duty cycle. PA1 Establish minimum engine and deteriorated engine thermodynamic conditions. PA1 Perform a transient heat transfer analysis using the thermodynamic conditions. PA1 Perform a transient finite element stress analysis using the heat transfer results. PA1 Establish the maximum operational strain ranges in the component for several locations, accounting for a "mission mix" of ambient conditions and engine deterioration. PA1 Determine the minimum fatigue life based on the strain ranges and existing specimen fatigue data. PA1 Apply a safety factor to the minimum life based on service experience, test experience, etc. to determine the retirement limit of the component. PA1 Inaccurate heat transfer or stress analysis. PA1 Improper duty cycle definition or operators who employ duty cycles other than that assumed. PA1 Hardware failures in the control system which allow engine operation at other than the assumed thermodynamic conditions. PA1 Control logic "bugs" which allow engine operation at other than the assumed thermodynamic conditions. PA1 1) the probabilistic distributions of the fatigue indication occurrence and fatigue failure life, PA1 2) a probability of detecting a detectible fatigue indication during inspection of any one of the given components, PA1 3) the forecasted components that will be in service during the first time increment, and PA1 4) the acceptable in service failure rate; PA1 1) collect fatigue data produced by the in service usage of the given components, and PA1 2) if a fatigue indication is detected in one of the given components during inspection, removing the one given component from service to prevent an in service failure; PA1 1) the revised probabilistic distributions of the fatigue indication occurrence and fatigue failure life, PA1 2) a probability of detecting a detectible fatigue indication during inspection of any one of the given components, PA1 3) the forecasted components that will be in service during the subsequent time increment, and PA1 4) the acceptable in service failure rate; and PA1 1) collect additional fatigue failure data produced by the in service usage of the given components, and PA1 2) if a fatigue indication is detected in one of the given components during inspection, removing the one given component from service to prevent an in service failure.
While theoretically ensuring a high degree of reliability, there are some disadvantages to the safe life method. For example, since the method is deterministic in nature and assumes minimum values throughout, the vast majority of components are forced to retire long before they have developed cracks or other failure indications. For the aircraft engine industry, this is not cost effective in that many engines are forced off wing and torn down to have hardware with remaining useful life removed and discarded.
In addition, experience has shown that despite the application of the safe life approach, fatigue failures in service can occur. For aircraft engines, this discrepancy is usually the result of one of several factors, including:
Also, since the declared safe life for a given component is calculated assuming no need for inspection, that component is generally not inspected prior to reaching its retirement limit. This is often the case even if the hardware is available for some other maintenance reason. If the original analysis on which the safe life limit is based should turn out to be non-conservative, valuable opportunities to detect negative fatigue trends in a fleet of engines are lost and oftentimes the first indication of a problem is an actual failure. Alternatively, if the analysis upon which the limit is defined proves to be overly conservative, a complicated and time consuming program of forced removals and inspections is required in order to gather data to support incremental life extensions.